This invention relates, in general, to pressure and acceleration sensors, and more specifically, to mechanical sensors using semiconductor material.
In the past, sensors designed for measuring forces due to pressure or acceleration, such as accelerometers, frequently incorporated deflectable diaphragms or cantilever beams. Recently, accelerometers have become available having cantilever beams which swing or oscillate above the channel of a field effect transistor (FET). The moving beam or diaphragm served as a gate of the FET so that current flow through the FET was controlled by mechanical movement of the beam or diaphragm. When such devices were implemented in semiconductor materials the mass of the moving beam or diaphragm was usually quite small and in order to achieve reasonable sensitivity additional weight had to be added to the moving part.
Usually, weight was added by depositing a heavy metal such as gold on to the beam or diaphragm to increase its mass. In the case of accelerometers, the additional mass increased deflection of the diaphragm or cantilever beam which resulted in greater current sensitivity of the FET to acceleration. One difficulty with such devices, however, is that the mass of the gate electrode is relatively small and therefore small deviations in any process which affected the mass of the gate electrode resulted in a large percentage effect on an output signal. Further, the additional materials required to form the weighted gate of the field effect transistor were rarely used in other IC manufacturing processes and so represented increased processing and cost for the accelerometer.
Designs which rely on oscillation of a cantilever beam or diaphragm obviously require a high degree of durability of the diaphragm or beam. In the past, these flexing structures have been manufactured from polysilicon and long term durability of such structures has always been questioned. Furthermore, the physical properties of polycrystalline silicon imposed limitations on size and shape of the flexing devices which were manufactured. These limitations imposed direct limitations on sensitivity and manufacturability of semiconductor accelerometers. It would be desirable to have a semiconductor mechanical sensor having high sensitivity which uses substantially only processes in a conventional MOS manufacturing facility while allowing a great deal of flexibility in design and manufacture of flexible, mechanical elements of the device.